1. Field of the Invention
The present invention relates to a downhole seal, and in particular to a downhole seal which incorporates a swelling material.
2. Description of the Relevant Art
It is often necessary to establish seals in downhole locations, such as in hydrocarbon exploration and production wellbores. In many cases seals must be established in annular areas, such as between a tubing string and a wall of the wellbore, for example an open bore wall or a cased or lined bore wall. Annular seals of the type described are conventionally identified as packers.
Many forms of downhole seals or packers are currently utilized which are arranged or mounted on the outer surface of a tubing string, such as a production tubing string or the like. Typically, the seals or packers are radially expandable such that they may be run into the wellbore while describing a reduced diameter, and then radially expanded to establish a seal at the required downhole location. Various arrangements exist for providing the required radial expansion. For example, seals may incorporate inflatable bladders which may be filled with a pressurized fluid. However, where high expansion ratios are required these inflatable bladders may become unstable, especially when exposed to large pressure differentials. Additionally, should the integrity of the bladder become compromised it may be difficult to maintain any form of seal.
Mechanical expansion arrangements exist which involve the axial compression of an elastic or otherwise deformable material to cause the material to extend radially. Such mechanically expandable seals, however, have limited capabilities when large expansion ratios are required. Additionally, actuation of such mechanical arrangements may involve complicated assemblies to ensure sufficient operation, and to ensure that axial actuation forces are efficiently and accurately converted to the required radial forces to establish the required seal.
US 2003/0079887 discloses a mechanical expansion arrangement in which top and bottom sealing rings are disposed on either side of a double-ramped cylinder. An end of each sealing ring includes a metallic structure and an elastomeric material, wherein the ends are arranged to be outwardly deflected by the double ramped cylinder into contact with an outer tubular to establish a seal. This known arrangement provides a combined elastomeric and metal-to-metal seal against the outer tubular.
Expandable seals which incorporate swelling materials are also known. Such seals normally comprise a band of swellable material, such as a swelling elastomer, mounted on the outer surface of a tubular body. When the swellable material is exposed to a particular activator, such as water, oil or the like, the material will radially expand. While such swelling materials can readily achieve large expansion ratios, it is understood in the art that the mechanical properties of conventional swelling materials diminish with increasing expansion or swelling. Thus, highly swollen materials are often considered unsuitable for downhole use.
As described above, many arrangements of expandable seals are known, although it is recognized that effective seals are very difficult to achieve where a high expansion ratio is required. This is a significant problem in the art as the architecture of a typical downhole environment normally requires a seal to be established in a large diameter bore, such as an overgauge or underreamed section, with access only provided through sections of a wellbore with relatively small internal diameters and restrictions. As such, seals which can accommodate such conventional downhole architecture and provide large expansion ratios are desired.